Toner has long been used as the developer for an electrophotographic image forming apparatus, such as a copying machine, a printer, etc. As the toner in an image forming apparatus is consumed for image formation, the image forming apparatus is supplied with the toner from a toner container (which sometimes is referred to as toner cartridge). Since toner is made up of microscopic particles, various methods have been proposed for preventing toner from scattering while supplying an image forming apparatus with toner, and some of these methods have been put to practical use. According to one of these methods, a toner container is kept in an image forming apparatus, and the toner in the toner container is discharged little by little from the toner container.
The black-and-white image forming apparatuses stated in Japanese Laid-open Patent Application 4-191770, 2002-351174, etc., are structured so that a toner container is mounted in the hollow portion of a photosensitive drum, in order to utilize the hollow portion of the photosensitive drum as the bay for a toner container to reduce the apparatus in size.
However, these image forming apparatuses are structured so that a toner container rotates with a photosensitive drum which is rotationally driven by the main assembly of an image forming apparatus. Therefore, it is possible that an image forming operation will be started even though there is no toner container in the apparatus, that is, even though the main assembly of the image forming apparatus is in the state in which toner cannot supplied to the main assembly. In other words, it is possible that the photosensitive drum begins to be rotationally driven even though the apparatus main assembly is in the state in which toner cannot be supplied to the apparatus main assembly. Further, in order to discharge the toner from a toner container, the toner container must be provided with some kind of mechanism for conveying toner in the toner container by receiving rotational driving force transmitted from the photosensitive drum.
In recent years, colorization has been rapidly gaining momentum in the field of an image forming apparatus, such as a copying machine, a printer, etc. Therefore, a color image forming apparatus employing multiple photosensitive members has begun to attract attention. Obviously, a color copying machine, a color printer, etc., use at least four developers different in color, for example, black (Bk), yellow (Y), cyan (C), and magenta (M) developers. Therefore, a space large enough for four toner containers is necessary in the main assembly of a color image forming apparatus. Thus, a color image forming apparatus is likely to be larger than a black-and-white image forming apparatus. In other words, the space required for the multiple toner containers is one of the primary obstacles that prevent an electrophotographic color image forming apparatus from being substantially reduced in size.